cmd/compile: de-virtualize interface calls

With this change, code like

    h := sha1.New()
    h.Write(buf)
    sum := h.Sum()

gets compiled into static calls rather than
interface calls, because the compiler is able
to prove that 'h' is really a *sha1.digest.

The InterCall re-write rule hits a few dozen times
during make.bash, and hundreds of times during all.bash.

The most common pattern identified by the compiler
is a constructor like

    func New() Interface { return &impl{...} }

where the constructor gets inlined into the caller,
and the result is used immediately. Examples include
{sha1,md5,crc32,crc64,...}.New, base64.NewEncoder,
base64.NewDecoder, errors.New, net.Pipe, and so on.

Some existing benchmarks that change on darwin/amd64:

Crc64/ISO4KB-8        2.67µs ± 1%    2.66µs ± 0%  -0.36%  (p=0.015 n=10+10)
Crc64/ISO1KB-8         694ns ± 0%     690ns ± 1%  -0.59%  (p=0.001 n=10+10)
Adler32KB-8            473ns ± 1%     471ns ± 0%  -0.39%  (p=0.010 n=10+9)

On architectures like amd64, the reduction in code size
appears to contribute more to benchmark improvements than just
removing the indirect call, since that branch gets predicted
accurately when called in a loop.

Updates #19361

Change-Id: I57d4dc21ef40a05ec0fbd55a9bb0eb74cdc67a3d
Reviewed-on: https://go-review.googlesource.com/38139
Run-TryBot: Philip Hofer <phofer@umich.edu>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/gc/subr.go

@@ -1679,7 +1679,6 @@ func structargs(tl *Type, mustname bool) []*Node {
 //	rcvr - U
 //	method - M func (t T)(), a TFIELD type struct
 //	newnam - the eventual mangled name of this function
-
 func genwrapper(rcvr *Type, method *Field, newnam *Sym, iface int) {
 	if false && Debug['r'] != 0 {
 		fmt.Printf("genwrapper rcvrtype=%v method=%v newnam=%v\n", rcvr, method, newnam)
@@ -1715,6 +1714,7 @@ func genwrapper(rcvr *Type, method *Field, newnam *Sym, iface int) {
 	fn.Func.Nname = newname(newnam)
 	fn.Func.Nname.Name.Defn = fn
 	fn.Func.Nname.Name.Param.Ntype = t
+	fn.Func.Nname.Sym.SetExported(true) // prevent export; see closure.go
 	declare(fn.Func.Nname, PFUNC)
 	funchdr(fn)
 
@@ -1918,6 +1918,14 @@ func implements(t, iface *Type, m, samename **Field, ptr *int) bool {
 		}
 	}
 
+	// We're going to emit an OCONVIFACE.
+	// Call itabname so that (t, iface)
+	// gets added to itabs early, which allows
+	// us to de-virtualize calls through this
+	// type/interface pair later. See peekitabs in reflect.go
+	if isdirectiface(t0) && !iface.IsEmptyInterface() {
+		itabname(t0, iface)
+	}
 	return true
 }